You are correct. The Tekno mounts are designed to accommodate motors of different diameters. 44mm are for finned Neu motors. 40mm for smooth can Neu motors. And 36mm for Feigao and Medusa motors. Obviously, if you get the 36mm you will be limited to Feigao an Medusa. You can use a 44mm mount to mount a smaller motor though. It wouldn't be ideal though as you would have a gap of dead air around the motor which could affect cooling. The Tekno mounts are an aluminum finned design engineered to help dissipate heat.
As far as ESCs - the best bang for the buck right now is the Castle Mamba Monster Max (MMM)
. Castle looks to have finally worked out the bugs since the initial release last summer. IMO nothing beats the MMM in ease of setup, configurability and manufacturer support.
If you have any other questions don't hesitate to call the guys at ImpactRC / TeknoRC. They are always more than willing to help out. Have fun with the build.